Gas turbine engine control systems are well known in the art. These control systems are specially configured for use with a particular civilian or military aircraft application. For example, fighter aircraft must be capable of undergoing violent maneuvers which require changes in engine power with corresponding changes in engine thrust to severely accelerate or decelerate the aircraft. To perform these maneuvers, the pilot must execute sudden power lever movements usually referred to as "bodies", "chops" or "snaps". These power lever movements produce extreme engine speed, temperature and air flow excursions. Engine controllers for fighter aircraft must provide maximum engine response as quickly as possible.
An example of an engine used in fighter aircraft is the F100 engine manufactured by Pratt and Whitney Aircraft, a division of The United Technologies Corporation, the assignee of the present application. The F100 engine is a multiple spool axial flow turbine power plant having a fan jet engine configuration. The engine is characterized by a fan or low compressor coaxial with a high compressor rotor. Both the fan and high compressor have vanes whose angles are adjustable while the rotor blades are moving. The engine also has a variable area exhaust nozzle.
During operation, the rotor speeds of the fan and high pressure compressor rotors will vary from a high speed (intermediate or military power) to a low speed (part power or idle power). To accomplish the dramatic changes in engine power output, not only are the fan and high compressor vanes changing position with the speed of the respective rotor, the variable exhaust nozzle is also changing area, as the amount of fuel provided to the engine combustor varies from a high value to a low value. For example, a "chop" in desired thrust would schedule the engine to reduce power from a military power condition, e.g. 12,000 lbs. of thrust, to 4,000 lbs. of thrust by reducing the amount of fuel to the combuster of the engine and increasing the exhaust nozzle area. The fan and high compressor rotor speeds, turbine temperature and air flow will decrease in accordance with the corresponding thrust profile. Similarly, there is a significant increase in rotor speed, temperature and air flow when the power plant undergoes a transition from part power to military power.
With existing jet engine controls there is a "bootstrap" process which occurs when there is a request for more or less power. Known controllers initiate a request for more or less power by increasing or decreasing fuel flow in response to the change in throttle power lever angle (PLA). The change in fuel flow produces changes in power via combustor exit conditions. Only in response to these changes are the engine spool speeds changed and, in turn, is the compression system geometry adjusted, such as the position of the fan variable vanes (FVV), high compressor variable vanes (HCVV) and exhaust jet nozzle area (AJ). In transient power situations the result is lessened engine flexibility and slower engine response. In addition, systems which control the gas path variable engine parameters in response to changes in fuel flow are characterized by non-optimum engine performance, as evidenced by higher fuel consumption and potential compressor system instability.